1. Field of the Invention
The invention relates generally to microwave amplifier assemblies and more specifically to methods and apparatus for modular assembly interconnections that have a low Voltage Standing Wave Ratio (VSWR) at eighteen GHz and above.
2. Description of the Prior Art
Gallium-arsenide (GaAs) field-effect transistor (FET) microwave amplifiers which cover the 0.5 GHz to twenty GHz band, and with output powers up to to two watts, are available from several manufacturers including Watkins-Johnson, Celeritek, and Avantek. Both balanced and feedback techniques are used with thin-film MIC and MMIC construction to achieve broadband performance. Typical amplifier designs are qualified and operated in the environments of MIL-STD-883 (space), MIL-E-5400 (airborne), MIL-E-16400 (shipboard), and MIL-E-4158 (ground). Some units are even tested to Hardness Assurance Lot Acceptance Test (HALAT) for space and strategic radiation dose tolerance levels.
Watkins-Johnson (Palo Alto, CA) manufactures ultra wideband small signal and power amplifiers for the 2-18 GHz band using a "matrix amplifier," which is reported in the March 1987 IEEE MTT Transactions. See, K. B. Niclas, R. R. Pereia, A. J. Graven, and A. P. Chang, "Design and Performance of a New Multi-octave High-gain Amplifier," 1987 IEEE MTT-S International Microwave Symposium Digest, pp.829-832. MMIC's and replaceable connectors are typically used in such amplifiers.
Celeritek (San Jose, CA) sells a line of "balanced amplifier" designs, models CMA and CMT, manufactured with MIC technology. The balanced design is promoted by Celeritek to improve interstage matching so several gain stages can be cascaded to achieve high gain while maintaining flat gain response. Three versions of amplifiers are available for operation from 18-26 GHz, 26-40 GHz, and over the full 18-40 GHz bandwidth. Units are assembled with interchangeable gain modules, allowing users to choose the amount of gain needed for particular applications. Two types of Celeritek FET's are used in the amplifiers, 150-.mu.m devices that yield 5-dB gain, and 300-.mu.m devices that deliver about 4-dB gain per stage. Cases for the CMT and CMA amplifiers use welded seals, glass-to-metal feedthroughs, and "K" type field replaceable connectors, with 2.4 mm coaxial connectors and waveguide interface connectors available as options.
Avantek (San Jose, CA) produces a line of wideband millimeter amplifiers known as the AMT/AWT series. This series performs over octave (AMT) and multi-octave (AWT) bands. The Avantek IK series packaging is a hermetically-sealed machined aluminum housing having optional waveguide and field-replaceable three millimeter coaxial connectors. The case length varies depending on the amount of gain and number of functions included.
In the prior art, it has been difficult to design amplifiers and subsystems using "modular construction" at frequencies above eighteen GHz, due to RF matching problems between modules. ("Modular construction" means building a unit meeting complex specifications from a supply of independently assembled, tuned, and tested modules designed to perform simple generic functions.) Modules consisting of substrates and chip components mounted on a metal carrier have problems maintaining a fifty ohm impedance in the region between modules. (The metal carrier is the mechanical and thermal base, as well as the RF ground plane.)
FIGS. 1(a)-(d) illustrate a typical coax case, referred to by the general reference numeral 10. The case 10 has a housing 11, an input coax connector 12, an output coax connector 14, and a DC power input 16. In FIGS. 1(b) and 1(c), a series of three amplifier modules, 18, 20, and 22, are inside housing 11. Module 18 is wire bonded to coax connector 12 and module 20 with wire bonds 24. Modules 20 and 22, and coax connector 14 are similarly interconnected with wire bonds 24. Modules 18, 20, and 22 may comprise dielectric substrates on metal carriers or may be just the substrate itself. In either event, the modules 18, 20, and 22 are bonded to the floor inside housing 11.
In FIG. 2, the wire bond 24 connections between modules 18, 20, and 22 are shown in greater detail. In the prior art, wire bonds 24 are ordinary round gage bonding wire, although ribbon or mesh are also used. The wire bonds 24 simply span over the space between the modules 18, 20, and 22. Ground paths are represented by arrows 26. Each of modules 18, 20, and 22 are comprised of a substrate 28 and a carrier 30. Above eighteen GHz, the prior art construction method shown in FIG. 2 will no longer operate satisfactorily. The wire bonds 24 have a prohibitively high impedance due to the air between modules acting as a dielectric, the small wire bond 24 size, the distance to ground in housing 11, and the distance between substrates 28. Multiple ribbons or wire bonds 24 reduce the impedance somewhat, but it remains well above fifty ohms. The ground paths 26 are electrically very long and cause problems, including that of having to optimize the thickness of carriers 30 to keep ground paths short. Thinner is better for the ground path, but too thin a carrier 30 can lead to substrate 28 cracking because carrier 30 is not a rigid enough support. In some cases, the carriers 30 have been eliminated, and the substrates 28 attached directly to housing 11. This shortens the ground paths 26 somewhat, but the impedance of the wire bonds 24 remains high. One problem with attaching substrates 28 directly to housing 11 with various solders is if any one module 18, 20, or 22 is defective, removal and replacement of that module is not practical and the entire assembly of case 10 is also defective and is wasted. If conductive epoxy attachment is used, modules may be removed, but the conductivity of the epoxy joint at the substrate 28 to housing 11 interface becomes critical and is difficult to manufacture reliably.